The need exists in many fields of application to measure measuring points of a surface of objects, and thus the objects themselves, with high accuracy. This is true, in particular, of the manufacturing industry, for which the measurement and inspection of surfaces of workpieces is of great importance.
There exists for these applications a range of measuring instruments that are designed for special tasks and are mostly denoted as coordinate measuring instruments or machines. These measuring instruments measure the surface for the establishment of a mechanical contact with the measuring point to be measured. Examples of this are portal measuring machines such as are described in DE 43 25 337 or DE 43 25 347, for example. Another system is based on the use of an articulated arm whose stylus arranged at the end of the multipartite arm can be brought into contact with the measuring point of a surface. Generic 3D coordinate measuring articulated arms are described in U.S. Pat. No. 5,402,582 or DE 1 474 650, for example. Comparable systems, also termed “Articulated Arm” or “Portable CMM” are, for example, piloted by “Romer” as “Sigma”, “Flex” or “Omega”, and by “CimCore” as “Infinite” or “Stinger”.
3D coordinate measuring articulated arms have a base, which is known in a reference coordinate system and fixedly positioned as one end of the articulated arm, and an opposite, movable measuring end on which the stylus is arranged. A tactile probe that consists, for example of a ruby ball that is mounted on a measuring rod can be used as standard stylus. Also alternatively known as probe elements are optical sensors that can be designed, for example, as a point gage or as a scanner, that is to say as scanning an object surface continuously and, for example, in a linewise fashion. Particularly, triangulation sensors can be used as such optical sensors. Furthermore, for example European patent application No. 07124101.2 describes the use of a camera as probe element that is designed to record or acquire a measurement object surface and is mounted on the movable end of an articulated arm. The position and alignment of the camera in space can be determined with high precision with the aid of the articulated-arm coordinate measuring machine.
A plurality of members or arm sections that can be pivoted and/or rotated relative to one another and are connected displaceably in relation to one another, if appropriate, are arranged between the two ends of the articulated arm such that the measuring end with the stylus, which is denoted as the probe member, can move freely within a portion of space. To this end, the members of the arm are interconnected by means of rotary, spherical and/or swivel joints as well as, if appropriate, by means of suspensions that enable a linear displacement. Furthermore, the articulations and/or suspensions are assigned position measuring devices such that it is possible in each case to measure a position of the members relative to one another, that is to say a relative position can be measured in each case between the members. For example, use may be made to this end of optoelectronic goniometers and optoelectronic length gages—as well as, particularly, optoelectronic position transducers that are designed to determine a relative position of members connected by means of a spherical joint.
The position of the stylus—and thus the position of the measuring point with which the stylus makes contact—relative to the base can be determined and specified in the reference coordinate system given the knowledge of an instantaneous measurement setting of the members, that is to say the respective positions of the members relative to one another, as well as of one of the members relative to the base. The determination of the position is generally performed by an evaluation unit that acquires the measured variables measured by the respective position measuring devices and derives the measuring point position therefrom. For example, computers or arithmetic logic units programmed particularly for this application can be used for this purpose.
In the case of such coordinate measuring machines, in particular in the case of articulated arm systems, it is usual to determine measuring points by using the individual measurement method, a measurement of the instantaneous member position being performed from each measuring point with which contact is made.
Although the use of high precision optoelectronic position measuring devices suffices to achieve high accuracies in the determination of the coordinates of a measuring point, many fields of application require a yet higher accuracy and, in particular, higher reliability for the determination of position.
For example, the friction in the articulations can lead to sagging of the connecting elements, and thus to a measuring error. Furthermore, friction in the bearings of the articulations can also result in occurrence of the so called slip-stick effect, as a result of which measuring errors can also occur. The term slip-stick effect in this case denotes the sliding back of solid bodies moving against one another, a sequence of movements comprising sticking, bracing, separation and sliding away being executed in each case.
Publication text WO 98/08050 discloses an articulated-arm coordinate measuring machine that has shock detectors and/or temperature sensors for the minimization or compensation of measuring errors.
A further known method for increasing the accuracy and the reliability of point measurements by coordinate measuring machines provides for the point that is to be measured not only to experience contact once, but for a second and, if appropriate, third control measurement to be carried out by removing and reapplying the stylus to the measuring point, that is to say by a new, decoupled measurement operation for the same measuring point. This method permits the detection of gross measuring errors of a point measurement, for example by a comparison of measuring point positions determined for one and the same measuring point—with the aid of measurement operations carried out separately. However, the comparatively large outlay of carrying out a control measurement proves to be disadvantageous in this case. There is likewise the risk that the stylus does not bear against exactly the same measuring point during a control measurement, and thus that an originally accurate, first measurement is corrupted as a result of the control measurement and leads to an inaccurate determination of position.
It emerges, furthermore, that—by contrast with the carrying out of only a single measurement—the accuracy with which the position of a measuring point is determined can be improved not at all or only to a certain extent, by carrying out one or two control measurements.